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When do information retrieval systems using two document clusters provide better retrieval performance than systems using no clustering? We answer this question for one set of assumptions and suggest how this may be studied with other assumptions. The “Cluster Hypothesis” asks an empirical question about the relationships between documents and user-supplied relevance judgments, while the “Cluster Performance Question” proposed here focuses on the when and why of information retrieval or digital library performance for clustered and unclustered text databases. This may be generalized to study the relative performance of m versus n clusters. © 2005 Wiley Periodicals, Inc.